Hello kilmac24 and welcome to the site.
There was a Thomas Whelan, member of C.Company 3rd Battalion, Dublin Brigade, he was executed on 14th March 1921 for his involvement in Bloody Sunday and is well documented.
Also the following Whelan,s made Witness Statements, which are held at the National Archives, Bishop Street , Dublin
J.Whelan, Waterford
Patrick J. Whelan, Cork
Patrick Whelan, Limerick
Patrick Whelan , Waterford
Sean Whelan , Wexford
William Whelan , Dublin
I cannot say if any of the above are related to you, but you could check them out, as I have said before, keep at it
Good luck
CSM